Fix Unicode 1.1 emojis

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/29557

When using the full font-stack some the Unicode 1.1 emojis are taken up
by fonts closer up the stack and don't represent the emoji well.

```
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto,
Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif, "Apple
Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
```

 - On Windows, "Segoe UI" -> "Arial" takes over
 - On mac OS, "Helvetia" -> ".SF NS Display" takes over

mac OS 10.12.3 | Windows 10
--- | ---
![](http://i.imgur.com/JI99zoH.png) | ![](http://i.imgur.com/aek8G1X.png)

---

When using a shorter emoji font-stack, everything seems to be covered
well 

```
font-family: "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
```

 - On Windows, "Segoe UI Emoji"  takes over
 - On mac OS, "Apply Color Emoji" takes over

mac OS 10.12.3 | Windows 10
--- | ---
![](http://i.imgur.com/bDwpGCw.png) | ![](http://i.imgur.com/7kUwEJQ.png)

app/assets/stylesheets/framework/emojis.scss

@@ -2,5 +2,6 @@ gl-emoji {
   display: inline-block;
   display: inline-flex;
   vertical-align: middle;
+  font-family: "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
   font-size: 1.5em;
 }